PHOSPHORESCENCE BY HEAT. 19 



when heated ; nor do most of the so-called chlor- 

 apatites, which contain chloride of calcium substi- 

 tuted in part or wholly for the fluoride. 



Of all these substances, the most remarkable 

 is fluor-spar (fluoride of calcium fig. 3). When 



Fig. 3. 



thrown in the dark upon heated mercury, into 

 boiling water, or on to a hot shovel, this mineral 

 immediately emits a brilliant phosphoric light. 

 Some specimens possess this property to a greater 

 extent than others. A certain green variety of 

 fluor-spar called Chlorophane becomes phospho- 

 rescent at the low temperature of 20 or 25 (cen- 

 tigrade), which is almost that of our summers. 

 Rare descriptions of chlorophane become phos- 

 phorescent in a dark room from the mere warmth 

 of the hand. According to Landrin (Diet, de 

 Mineralogie) some varieties are almost constantly 

 luminous in the dark.* 



* Fluoride of calcium loses its phosphoric property after it 

 has been once heated. Miller asserts ('Elem. of Chemistry') that 

 when a phosphorescent fluoride of calcium is dissolved in hydro- 

 chloric acid, and then precipitated by ammonia, the precipitate is 



